I’m wondering a bit about the physics and biology of hand slings. Using them is one of my hobbies, and I’m always really impressed at how fast I can adapt to a different sling or a different style or different ammunition.

From what physics knowledge I’ve got, it seems like you’d need quite accurate release timing for a given throw, and a different slings, styles, and ammunition would need different timing. I’m wondering about actually how accurate release timing needs to be and how it must vary based on projectile mass, sling mass, sling length, and throwing style (not to mention other stuff like stiffness/stretchiness of the sling). I’m also wondering how I’m able to adapt so fast — it’s literally 2 or 3 throws, and it’s like I’ve recalibrated my throwing to the new style and or sling. It’s also always interested me how I can tell when to release — especially considering what a small window the release needs to be in to get a projectile to go forward (I mean, my arm is ~1m long, as is the sling. So the ammunition is whirling around a ~1.5m circle and going quite fast. And yet, I’m able to release with enough consistency to nail a 1.5m x 1.5m target 30m away maybe ~60-70% of the time using random rocks I pick up.)

You have very reliably learnt to feel the action of release? By detecting the push-back before/during/after release and 'knowing' what should work you unconsciously microadjust your action to account for distortion, once you get the initial general feel of the differences, and having trained nerve/muscle-memory to shortcut the more conscious relearning mechanisms of a relative amateur.

(And, for the general feel, it's probably like having to adapt to walking whilst carrying an awkward load, the different characteristics of your bodily system (including the different 'bodily extension') may cause you a few steps of re-learning the basic movements needed but you quickly establish a sweet-spot that means you don't walk sideways/aim off, or rather that you change so that you may still do so but by 'trying' to go in the other direction from the forced-error you come back to the original intention.)

Edit: Also, from videos of the competitions described there, the swing is much slower than 8 revolutions per second.

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Let's see -- I'm not actually sure how I aim. It has to do with the angle of my hand and knowing when to release, but I sort of do the proper adjustments automatically. I definitely have learned to "feel" when to release as mentioned earlier.

Regarding velocity, it's not all that hard for me to throw a rock the size of a large egg (usually limestone, so ~100g or so) about 100m with a sling if I'm going for distance (so release way earlier than I normally do at maybe ~45 degrees) -- which puts the velocity at something like 30 m/s (given no air resistance, which I'd imagine is a big factor). So, 40 m/s is a reasonable number for the launch velocity (especially given air resistance and the fact that those 100m throws aren't anywhere near the furthest I can toss and that I'm not a great slinger).

The throwing style I mostly use -- figure 8 (not my video) -- has only a single rotation around the head. The radius of that rotation is probably ~1.5 meters (my forearm plus the folded sling), making it more like 4.25 revolutions per second.

The competition you talk about is in the Balearic Isles and is generally regarded to be pretty much the standard. On that kind of target, I can hit the target about 70% of the time at 20m, with hitting the bullseye maybe 15% of the time. But, I've never tested that with consistently shaped ammunition -- only with random rocks ranging from maybe nickel sized to fist sized. And I don't consistently practice -- I maybe practice a couple times a week for an hour or so, with long, long breaks between "active" times and times when I don't practice at all.

A 50 cm bullseye at 20 m subtends an angle of about 0.025 rad (thank you, small angle approximation), so if the stone is moving at 4.25 rotations per second = 26.7 rad/s, that gives you about a 0.94 ms release window. Since you obviously don't get the bullseye every time, your actual timing is not quite that precise, but it should be within an order of magnitude. Millisecond-precision timing is very impressive, and this type of thing is also necessary for throwing accurately at long distances. It is something we as humans seem to be almost uniquely good at.

Eebster the Great wrote:Millisecond-precision timing is very impressive, and this type of thing is also necessary for throwing accurately at long distances. It is something we as humans seem to be almost uniquely good at.

Humans are also very good at recognising and following patterns. I reckon that by getting a regular swing going, it is easier to mentally home in on the point of the swing where the release should be, like a musician joining in with the rest of a band. If you had to release in the first or second swing, it would be much more difficult.

Edit: Also, from videos of the competitions described there, the swing is much slower than 8 revolutions per second.

Sorry, yes. 0.017 radians.

For small enough angles, just treating the target width as the opposite and range as adjacent is close enough, but I prefer to be accurate, so I halved the width, calculated half the angle, doubled it, realised the calculator was in degrees, started again and forgot to double it the second time through.

Is a "Contraception App" really contraception if it just tells you when to use a contraception*? Whether it's accurate or not (and however much it is guessing or not) when it tells you to use contraception you're then supposed to use one of the forms of contraception that it seems to be claiming to be better than, thus already reducing its efficacy to the level of that thing you haven't been using when it doesn't ask you to.

"To bring light back to the gardens of Moria, the Dwarves crafted great mirrors, and placed them atop the summit of Zirak-Zigil."

Assuming they can manage airflow well enough to keep the air cave-system klar rather than forge-ceiling schmutzig, what's the curve like on the graph of whether it's more efficient to do that or to cover the summit in PV panels and run LED lights in your garden?

I always imagined it was something like a Light Pipe. Or a set of such shafts, specifically with mirror-assisted capture either as a heliostat or a static 'whole-sky concentrator' to act as a sort of fish-eye lens across the track of the sun mapping to the shaft.

(Possibly crystal-filled, if they had the means to work or even grow such a thing, for the total-internal-refraction method. But I imagine highly polished metal lining would suffice for the mirrored version.)

I don't know if JRR would have known about/imagined full tubes of this kind, as seen these days, but Deck- and Pavement-Lights aren't a new thing.

The rest is just a matter of having as much practical collection area atop as effective dispersal area below (the refraction/reflection within the shaft(s) smudges out the bright-spot, I imagine) and daylightesque illumination, not much more/not much less, could 'feed' the gardens.

The big issue might be the energy concentration through the tube(s) themselves, at least in whatever absorption bands the system suffers with. Maybe they have a mithril alloy ("It could be beaten like copper, and polished like glass; and the Dwarves could make of it a metal, light and yet harder than tempered steel. Its beauty was like to that of common silver, but the beauty of mithril did not tarnish or grow dim.") that suited the purposes. Though I could imagine a warming of the air that would turn the tubing into a powerfully-draughting chimney (it could aid with the ventilation?) if not capped with baffled and/or sealed vanes/windows.

I have had in mind for a while now a lunar-base design where the agriculture is undertaken in underground areas (dug out voids, dug-down-to natural voids, cut'n'covered trenches in regolith or surface/shallow-trench-set modules where regolith is piled atop) with light-tubes set down to them. The tubes would be kinked (like a periscope, but halved and remounted eyepiece to forward lens) to stop direct radiation, the mirror/prism system designed to pass 'safe' radiation around the kink, and at the top either a heliostat or broad capture device. Given the 28-day solar cycle, WRT the Moon, it would have to be augmented for lunar night (backup illumination) and if the moon-daily cycle were too long in terms of daylight a grid/diffraction-grating could block (or pass around amongst multiple light-tube heads, destined to illuminate different 'fields' below) the normal light on a 12ish hours on 12ish hours off cycle.

Or a PVPanel->Cells->Timer->LED system could be used, but I'd been imaging this more as a passive system (for the setting I'd been thinking of) that at least during the two weeks of natural daylight would be self-regulating. Either arctic/genetically-made-arctic-like plants could be used that fully tolerate the extreme day/night lengths or rapid-growth-cycling plants could be raised within the 14 days of illumination and the system (hydroponics/aeroponics?) left fallow in the fortnight gap between harvest and reseeding. Might not work well with an aquaculture system of fish+plants.

But I had gone so far as to imagine some passively automated system (bimetallic levers tuned to a secondary solar-heat detector?) to cap superfluous tube-tops. During lunar-day the caps would rotate over tube-tops, covering and uncovering them on a 24-hour 'winding' cycle. Dumb-as-a-brick hardy PV/storage systems atop the caps would make some inefficient use of the incident light not destined to go down that tube. Passing into lunar-night, the mechanism whirs back and the (independently detected) lack of solar power triggers LEDs or other means of electoluminescence set within the underside of the 'caps' to send a smattering of the 'surrogate' solar light down the various shafts in lieu of the Real McCoy.

(But that was intended as a high-science/zeerusted setting where lack of technical maintenance was no bar to some 'reasonable' continuing functionality of a well-planned Moonbase. Also considered was a whole equatorial-ring complex where the entire lit half of the complex would directly supply as much or as little of the whole ring as needed, whether or not the locations needing energy had the Sun currently risen above their own set of PV panels. Obviously needs something like an HT-cabling running round the whole circumference along with the continuous/intermittent PV topping.)

Back to Moria (and away from recalling my prior less-than-Fleeting Thoughts about the other thing), some of the Lunar Base problems are moot to their design. I think that suitably-designed mirror-collectors, mirrored tube-linings and perhaps some method of further diffusing the semi-collimated beams punching through the ceiling back up onto a suitable 'rock sky' surface could (within the realms of the arcane workings of Middle Earth and its 'physical'-cum-magical everyday accessible properties) suffice to create a subterranean Eden. Give or take the active labour of dwarven mechanics to keep the system going prior to the desertion of the underground kingdom.

I was looking forward to hearing David Baddiel trying to understand WiFi, Maybe we'd learn something about Hedy Lamarr and her torpedoes, in the necessarily highly edited-down 15 minute version of the slot.

*cringe*

Even with David's lack of understanding as a starting point, I think I class that final explanation as a fail. We're left somewhat unsure if WiFi can work in a vacuum, and nobody really addresses the briefly raised EM radiation/atomic radiation issue. On air, at least. Then there's 'speed' in terms of bandwidth and latency, completely glossed over.

(Is that audio accessible globally? Apologies if not and you're outside the UK. Maybe for the best, though, just to imagine what my frustration is about.)(ETA: Going to the page I just blindly linked, it's unavailable even to me, at the moment. I was listening to a 'live recording' I made last night. I assume it will become available at some point, unless they're really embarrassed about its content and have blocked it until/unless they fix it.)

No (well, apart from gamma being at the other side of the EM spectrum from radio, which was momentarily covered equally badly). But the programme¹ briefly had some sort of "is wifi radiation like nuclear radiation?" pondering but never got around to clarifying that it wasn't. It was a mess, honestly. Hence my chagrin about the whole thing.

¹ "Sorry, not available" still, on that link, strangely. It currently promises that after tonight's programme on Fashion is broadvadt that will be available. Yet to be tested though. I have still got the sneaky personal copy I made off the air to time-shift listen to it, but sharing that would be not easy or condoned by TPTB. Maybe I should write it up as a transcript?

Soupspoon wrote:No (well, apart from gamma being at the other side of the EM spectrum from radio, which was momentarily covered equally badly). But the programme¹ briefly had some sort of "is wifi radiation like nuclear radiation?" pondering but never got around to clarifying that it wasn't. It was a mess, honestly. Hence my chagrin about the whole thing.

¹ "Sorry, not available" still, on that link, strangely. It currently promises that after tonight's programme on Fashion is broadvadt that will be available. Yet to be tested though. I have still got the sneaky personal copy I made off the air to time-shift listen to it, but sharing that would be not easy or condoned by TPTB. Maybe I should write it up as a transcript?

Holy Heck.

It sure sounds like a piece of media that should never have seen the light of day. Why on Earth would you want to preserve it?

ijuin wrote:Bah, any EM radiation longer than ultraviolet-A is virtually harmless at intensities below several watts per square meter.

I dont think wifi or mobile phones are harmful, but to be fair, its quite possible for the flux right next to your phone/router to be significantly greater than a few watts per square meter. Most phones can transmit between 1-3W at max power output, and radiated from a very small volume (ie: <<<1sq.m)

Your phone being pressed up against your brain for hours every day could plausibly be a hazard, but unless you walk around or sleep with your wi-fi hub against your head, then the risk from wi-fi radio waves is about a thousand times less than from your phone.

(There's still no reason why the episode remains unavailable, but looks like it's intentional. I suspect a contributor withdrew consent, in light of its total messiness. Or David himself, because I've now seen some if the flack that went his way.)

It is pointed out that the older-band of wifi signal is in the same range as a microwave('s leakage?) but vastly less power. Hence https://xkcd.com/654/ issues, of course.

I'm gonna write up a transcript, I think, just to put this to rest (after the transcript is picked over, maybe, to point out errors/omissions in my comprehension of what was said), but it'll be the weekend at earliest before I get to do it.

So, I'm not a professional transcriber, but for the record (and under fair-use reportage, but Mods can expunge this whole post if they don't think it is proper) here's that episode. It's a mess. Maybe of transcription (tried to be faithful to it) but, having now heard it myself several times more while putting it down as text, definitely in content.

The often very conversational style stuttering and mumbling is here. Ellipses for pause-to-halts not otherwise doable in punctuation, <things in pointy brackets> are commentary/query by me rather than direct speech, [in these type] I've set overtalk/cut-over speech from a person so labelled. Question marks may have been used to bracket edge-of-intelligible words, but I may have caught and sorted all those in a later pass, or shoved them into <>s.

"DB:" is David Baddiel. When it's not the same as the last time of speaking, clearly labelled as either narrating (overdubbed, in post-production) or by the 'live' location within which he spoke into the mike in the original locale of each segment (marked *THUSLY*, you'll note).

Everyone else is named (best I can, not sure about Bart's surname, not entirely sure his first name isn't actually Bharat or similar). I've done the "boffin" the honour of calling her "Dr TM" (and checked her name on the university website), rather than her first name. I've decided she was done up like a kipper. Badly edited, badly understood, maybe thought she was there to explain a different aspect of things and I'm sure a lot of her really useful information got left on the edit-room floor/bit-bucket. Though, even with those assumptions, there's a mystery over the 65%.

Only when I knew I was at leisure to leave my other possible procrastinations until later.

I'd only posted originally because I thought I could sufficiently back up my slight but significant frustration of the listening by anyone caring to suffer it themselves. Given how the conversation went, it seemed the best thing to do to justify it as an intrusion into the forum consciousness. It's there for those who want it. It's there for those who don't, too, but with no compulsion.

(15 minutes of audio, played at half speed to mostly keep up when typing. Maybe averaging no more four times that each bit played past, between starting over again to ensure consistency or hotkey jumping back a few seconds to pick up an awkward bit straight off the bat. Two hours, plus some trivial reformatting. I wasn't being paid to do it, but I wasn't being paid to do anything that evening. So it's no more a waste of time than a few other things I might have done instead. And healthily cathartic. Eventually!)

((Not sure if I'm open to further transcripts, on commission. I doubt I'd be efficiently accurate enough to compete with the existing pool of service-providers.))

I don't think it's worth dissecting, but now you know where I was coming from.

Sheesh. Yeah. When the explanation of electromagnetism still leaves him thinking that it involves air molecules vibrating, it's no surprise that he can't understand that there is a difference between the signal propagation speed and the bit rate.

(In this week's David Baddiel thing, he actually spent a paragraph apologising for the main WiFi error. And he seemed to blame the expert, whereas I think she might not have been quite so much to blame. - But that's not why I'm back here.)

The Kilogram to be redefined? (To a method involving electric current/force. Which, like the article, does rather confuse weight and mass, at least as briefly explained there.)

That seems to invoke a circular definition—any current is also going to have a specific voltage, and the volt is defined in terms of amperes and Newtons—the Newton itself being defined in terms of kilograms.

ijuin wrote:That seems to invoke a circular definition—any current is also going to have a specific voltage, and the volt is defined in terms of amperes and Newtons—the Newton itself being defined in terms of kilograms.

They're using the current* definition of the kilogram to accurately measure Planck's constant, then defining that as a fixed value and deriving the "new" kilogram from it. So in the end, it's not circular.

*as in what we have now, not electrical current

gmalivuk wrote:

King Author wrote:If space (rather, distance) is an illusion, it'd be possible for one meta-me to experience both body's sensory inputs.

Yes. And if wishes were horses, wishing wells would fill up very quickly with drowned horses.

Yeah, this is all a good thing. The kg was the last artifact-based physical unit; all the rest have already been redefined in terms of physical constants. Artifacts suck because they *change* (and there's evidence that the reference kg *has* been getting, very slightly, lighter over the 130ish years it's been in use).

The candela is still dumb though, because the "physical constant" it is defined by is actually not defined, and is an arbitrary function mapping frequency to intensity in typical human perception, for some arbitrary "typical human." I mean sure, the "luminous efficiency constant" is a constant for a particular frequency of light, but you still need a standard function for every other wavelength. I can't understand why the candela is a base unit at all, to be honest.

The mole is also now just a really big integer (specifically, 602,214,076,000,000,000,000,000). So again, it's not really a "base unit" any more than "thousand" is a base unit. The other units are cool, though. However, it does seem a bit more natural to define the coulomb as the base unit rather than the ampere, though it amounts to exactly the same thing in the end. And that's still not as annoying as calling the base unit of mass the "kilogram" rather than the "gram," though that historical accident is here to stay.

Why does SI have 7 base quantities? It seems to me that the mole is simply a scaling factor and the candela an even sillier scaling function. The only alternative I can find apart from CGS and MKS, though, is a rather shitty paper (https://arxiv.org/abs/0710.3483) that brings it down to 3 quantities: length, time and "chakr". But I don't understand from the text how the author got to 3 dimensions, much less guarantees that you don't end up with faulty combinations like in the given example (which has temperature=mass*length for crying out loud).In any case, if I understand correctly, temperature can be derived from energy (which can be derived from MLT) and the Boltzmann constant, while current can be derived from MLT and the permittivity/permeability constant. Then again, you can also extend this to deriving length from time and the speed of light and derive mass from length (and therefore time) and the density of carbon, so you're left with 1 type of dimension or base quantity.Does it make sense to ask how many base quantities our universe has?

If you are getting mass from the density of carbon and length, it's still a different dimension and depends on a physical constant (the density of carbon). At that point you might as well define it directly in terms of the mass of a carbon atom. Or the Planck constant. Using physical constants to define base units makes sense. We could avoid using units at all by constantly referring to quantities in terms of combinations of physical constants or ratios with other known quantities, which again, is not functionally different from using units.

But Avogadro's constant is silly because it does not relate two units, just one: the mole. So instead of defining everything in terms of the second and physical constants, we define almost everything in terms of the second and constants and one thing as an integer.

Flumble wrote:↶The only alternative I can find apart from CGS and MKS, though, is a rather shitty paper (https://arxiv.org/abs/0710.3483) that brings it down to 3 quantities: length, time and "chakr". But I don't understand from the text how the author got to 3 dimensions, much less guarantees that you don't end up with faulty combinations like in the given example (which has temperature=mass*length for crying out loud).

They did a search over a bunch of possibility space, where a computer assigned each of the five base units to some combination of three new base units, then checked a bunch of quantities with distinct unit signatures in the old system and verified they still had distinct unit signatures in the new system. Then they went thru all the ones that passed that automatic test and manually checked them for goodness, landing on the one they settled on with length, time, and "chakra". (For reference in the rest of this post, they called this 3-unit system "New Construction" or "NC".)

This actually does have some interesting qualities to it. All the vacuum constants and a bunch of electrical quanties, which contain a mass unit in SI despite not actually depending on mass, are expressed purely in terms of length and time in NC, no chakra at all. Charge, a fundamental quantity that is nevertheless expressed in terms of time in SI, has no time in NC, just chakra and length. Mass, rather than being a fundamental unit, ends up being expressed as (chakra/volume)*time³, expressing a semantic that mass actually does take up volume (and that singularities are thus indeed mathematical nonsense).

ijuin wrote:↶That seems to invoke a circular definition—any current is also going to have a specific voltage, and the volt is defined in terms of amperes and Newtons—the Newton itself being defined in terms of kilograms.

In practice, the new exact values for the Planck constant and the charge of an electron means it's not circular any more:

Current in a Kibble balance is measured by way of a resistor in the circuit. Resistance can be determined to about 1 part per billion using the von Klitzing constant, which describes how resistance is quantized (has specific numerical values) through a phenomenon called the quantum Hall effect. Voltage is measured using the Josephson effect (and its associated Josephson constant), which relates voltage to frequency in a superconducting circuit, with measurement uncertainties in the range of 1 part in 10 billion. The Josephson effect is the world's de facto standard for quantifying voltage.

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I'd say 1 counts as an SI "unit" (not that any number by itself is really a unit).

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Though the mole is another SI unit, so in that sense there's also 602,214,076,000,000,000,000,000 as a natural-numbered unit.

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